This invention relates to temperature sensing apparatus, and in particular to circuits and methods for temperature sensing.
For high power circuits such as power amplifiers for audio speakers and linear power supply regulators, there is a possibility of fault conditions such as external short circuits causing high on-chip currents. The on-chip power dissipation caused by these currents can result in excessive temperatures which can degrade the characteristics of circuits on the silicon chip and, in extreme cases, may even constitute a fire hazard. For this reason such power circuits are often provided with a thermal shutdown function where power outputs are disabled if the chip temperature exceeds a predetermined limit, for example 150xc2x0 C. To implement such a function an on-chip circuit is needed to detect and flag when such a predetermined temperature threshold is exceeded. There is also a need for a temperature detector in some microprocessor systems, for example where the microprocessor is clocked at a high speed. In such a system if a temperature limit is reached the clock may be slowed down to reduce the supply current drawn by the microprocessor and/or an output signal may be provided to turn on a fan.
In the early days a Zener diode voltage would be resistively divided and applied to the base of a common-emitter bipolar transistor. The base-emitter voltage (Vbe) to turn on a bipolar transistor decreases by approximately 2 mV per xc2x0 C. so that as the temperature increased with a constant voltage applied (or even a rising voltage if the Zener had a positive temperature coefficient or tempco) a temperature would be reached where the bipolar transistor turned on and its collector current could then be used as an output.
As supply voltages have reduced this method has become impracticable as typical Zener voltages, which are difficult to achieve reliably below 5 to 7V, are too large. Instead it has become conventional to use a bandgap voltage instead of a Zener voltage, as described for example in U.S. Pat. No. 3,959,713, U.S. Pat. No. 4,692,688, U.S. Pat. No. 4,574,205 and U.S. Pat. No. 5,099,381. For example U.S. Pat. No. ""381 describes a circuit where a bandgap voltage from a Brokaw cell is compared to a Vbe multiplier voltage. To avoid electrically and/or thermally induced instability about the threshold temperature some local positive feedback may also be applied to provide the switching point with some hysteresis. A temperature detection circuit employing a bandgap voltage source and feedback to provide hysteresis is described in U.S. Pat. No. 5,149,199. General background prior art in the field of temperature detection can be found in U.S. Pat. No. 6,181,121, US 2002/0093325, U.S. Pat. No. 6,188,270, U.S. Pat. No. 6,366,071, U.S. Pat. No. 5,327,028, U.S. Pat. No. 4,789,819 and U.S. Pat. No. 5,095,227.
The IEEE Journal of Solid-State Circuits, vol. 31, no. 7, July 1996, pages 933 to 937, xe2x80x9cMicropower CMOS Temperature Sensor with Digital Outputxe2x80x9d, A Bakker and J H Huijsing, describes a CMOS temperature sensor in which a current proportional to a Vbe voltage is compared to a reference current which is substantially independent of temperature formed by the addition of the PTAT (proportional to absolute temperature) current to a base-emitter voltage referenced current. The sum of these two currents is approximately temperature independent because they have opposite temperature coefficients, positive for the PTAT current and negative for the Vbe current. However the circuit of Bakker and Huijsing is relatively complicated (see, for example, FIG. 4) and its sensitivity could be improved.
Another temperature detection circuit is described in U.S. Pat. No. 5,980,106, which again uses a bandgap reference. FIGS. 1A and 1B, which are taken from U.S. Pat. No. ""106 illustrate the principle of this circuit. Broadly speaking two current sources 10, 20 with respective positive and negative temperature coefficient characteristics 12, 22 are applied to a detection node A coupled to an output circuit, in FIG. 1A inverter 30. As can be seen from inspection of FIGS. 1A and 1B the inverter output will switch where the voltage of point A crosses the switching threshold for the inverter, in FIG. 1B at threshold temperature TD. U.S. Pat. No. ""106 also teaches the application of feedback to detection node A as shown, for example, in FIG. 3A of ""106. A detailed temperature detection circuit (FIG. 4) is also described in which a thermal voltage (VT)-based current Ith is combined (compared) with a current derived from a bandgap reference Ibg at node A (negative temperature coefficients introduced by resistors in the circuit cancelling). Again, however, the circuit of U.S. Pat. No. ""106 is relatively complex and includes floating bipolar transistors as well as MOSFETs.
It is desirable to be able to provide a simpler, cheaper and easier to fabricate temperature sensor. A bandgap voltage is often present in circuits such as voltage regulators but is unnecessary in applications such as speaker amplifiers, so that an arrangement not reliant on an explicit bandgap voltage generator would be preferable. Furthermore, it has been recognized that fundamentally it should be possible to construct a temperature detector merely by comparing two quantities with different temperature coefficients and predictable absolute values, or at least with predictable relative values at some reference temperature from which temperature coefficients may be referred. Also, increasingly circuits are being manufactured using CMOS rather than bipolar technology, even in traditionally bipolar areas such as loudspeaker power amplifiers (see, for example, the Fairchild FAN 7021). The use of CMOS precludes the application of many prior art techniques.
According to a first aspect of the present invention there is therefore provided a temperature sensor comprising: a current mirror with an input and at least two outputs; a first reference current generator having a first current input and a first current output and configured to generate a first reference current with a positive temperature coefficient at said first current output in response to said first current input; a second reference current generator having a second current input and a second current output and configured to generate a second reference current with a negative temperature coefficient at said second current output in response to said second current input; and wherein one of said first and second reference generators has a respective current output coupled to said input of said current mirror; said first current input of said first reference generator and said second current input of said second reference generator share an input node coupled to a first of said current mirror outputs; and the other of said first and second reference generators has a respective current output coupled to a second of said current mirror outputs to thereby provide a current sense node; and wherein said first reference current generator comprises a thermal voltage referenced current source, and said second reference current generator comprises a temperature dependent semiconductor characteristic referenced current source.
In this specification the term current source includes negative current sources, that is sources in which a current flows into the source (sometimes alternatively referred to as xe2x80x9csinksxe2x80x9d), and current may therefore flow into a current source output. Broadly speaking, two reference current sources are provided, both interacting with the same current mirror, one of the current sources being referred or substantially proportional to a bipolar transistor base-emitter voltage (negative temperature coefficient), the other of the current sources being referred or substantially proportional to a bipolar transistor thermal voltage (in mathematical terms kT/q where k is Boltzman""s constant, T is the absolute temperature in Kelvin and q is the charge on an electron). Such a thermal voltage referenced source is sometimes referred to as a PTAT (proportional to absolute temperature) source although in practice if the output is extrapolated back to absolute zero there may be an offset.
This arrangement provides a particularly simple and elegant temperature sensing circuit with performance parameters which are relatively straightforward to determine and which can be made relatively consistent in practice. In a preferred embodiment the thermal voltage referenced source comprises a pair of bipolar transistors and one of these transistors also provides a base-emitter voltage to which the second current source can be referenced, providing a further simplification and locking the parameters of the two current sources together more closely.
The temperature sensing circuit is suited to fabrication in MOS, particularly CMOS technology and in this case the circuit is such that the bipolar transistors employed in the current sources may comprise parasitic (vertical or lateral) devices inherent in CMOS technology, typically vertical PNP transistors in P-substrate CMOS and vertical NPN transistors in N-substrate CMOS. The circuit may also be fabricated in BiCMOS.
In other embodiments the first (positive temperature coefficient) source may employ MOS rather than bipolar transistors for example using a xcex94Vgs rather than a xcex94Vbe-type arrangement, and the second (negative temperature coefficient) source may then comprise a MOS VT-referenced or low-current Vgs-referenced source.
In preferred embodiments the temperature sensor includes a positive feedback and this may be advantageously applied by injecting current into the shared input node. This positive feedback will tend to result in a switching-type behaviour at the current sense node output, so that as the output begins to change the positive feedback encourages this change. The positive feedback also provides hysteresis about a threshold switching temperature. In one embodiment the feedback may be provided by a form of differential amplifier or differential or long-tailed pair in which one of the transistors of the pair has an input from the current sense node and the other has an input connected to a suitable bias voltage. Preferably the sensor also includes an output circuit to provide an essentially binary output depending upon whether or not the temperature of the circuit (more particularly, of the bipolar transistors) is above or below the threshold, taking into account hysteresis.
In a related aspect the invention provides a method of providing a temperature dependent signal, the method using: a current mirror with an input and at least two outputs; a first reference current generator having a first current input and a first current output; a second reference current generator having a second current input and a second current output; and wherein one of said first and second reference generators has a respective current output coupled to said input of said current mirror; said first current input of said first reference generator and said second current input of said second reference generator share an input node coupled to a first of said current mirror outputs; and the other of said first and second reference generators has a respective current output coupled to a second of said current mirror outputs to thereby provide a current sense node; the method comprising generating, using said first current generator, a first, transistor thermal voltage referenced current with a positive temperature coefficient at said first current output in response to a signal from said current mirror at said shared input node; generating, using said second current generator, a second transistor voltage referenced current with a negative temperature coefficient at said second current output in response to said signal from said current mirror at said shared input node; and combining signals dependent upon said first and second reference currents at said sense node to provide said temperature dependent signal.
It will be appreciated that the combining of the signals may comprise either a comparison of the signals to one another or a subtraction of the signals from one another. The temperature dependent output signal (at the sense node) may comprise either a current or a voltage signal.
In another aspect the invention provides a temperature detection circuit comprising: a current mirror having an input and first and second mirrored current outputs, said input and said first mirrored output being coupled via respective first and second MOS transistor channels to respective first and second transistors to set a ratio of current densities in said first and second transistors to provide a positive temperature coefficient current from said second mirrored current output; a third MOS transistor having a gate connection coupled to a gate connection of said first MOS transistor and a pair of channel connections, one of said channel connections being coupled via a resistor to a common connection of said first and second transistors to provide a negative temperature coefficient current output at said other channel connection whereby said current output is referenced to a temperature-dependent voltage of said first transistor, said other channel connection being coupled to said second mirrored current output to provide a temperature dependent output.
In a related aspect the invention provides a temperature detection circuit comprising: a current mirror having an input and first and second mirrored current outputs, said second and first mirrored outputs being coupled via respective first and second MOS transistor channels to respective first and second transistors; a third MOS transistor having a gate connection coupled to a gate connection of said first MOS transistor and a pair of channel connections, one of said channel connections being coupled via a resistor to a common connection of said first and second transistors to provide a negative temperature coefficient current output at said other channel connection whereby said current output is referenced to a temperature-dependent voltage of said first transistor, said other channel connection being coupled to said current mirror input to provide negative temperature coefficient current from said second mirrored current output; and wherein a ratio of current densities in said first and second transistors determines a positive temperature coefficient current which is combined with said current from said second mirrored current output to provide a temperature dependent output.
In a one embodiment the positive temperature coefficient current is a current flowing in the first MOS transistor channel.
In the specific embodiments described later the first and second transistors are bipolar transistors, the first MOS transistor has its drain and gate connected together and the second MOS transistor has a resistor connected between its source and the second bipolar transistor. Each bipolar transistor, which may be parasitic in CMOS technology, has its base and collector connected together. A feedback circuit is preferably employed so that the temperature dependent output exhibits roughly bistable behaviour either side of a threshold temperature, with some hysteresis. Means may also be included to adjust the threshold temperature, for example by effectively adjusting said resistor (used to convert the first bipolar transistor base-emitter voltage to a current) and/or by effectively injecting current into or drawing current from said temperature dependent output.
In a further aspect the invention also provides a method of generating a temperature dependent signal, the method comprising: generating a thermal voltage referenced positive temperature coefficient signal using a pair of transistors operating at different current densities; generating a transistor voltage referenced negative temperature coefficient signal using the a transistor voltage of one of said pair of transistors; and subtracting one of said positive and negative temperature coefficient signals from the other of said signals to generate said temperature dependent signal, whereby the temperature dependence of said temperature dependent signal is greater than either of said subtracted signals.
Preferably the transistors are bipolar transistors and the transistor voltage is a base-emitter voltage. The use of thermal voltage-referenced and base-emitter voltage-referenced signals, preferably current signals, rather than a bandgap reference enables the same transistor to be used for both Vbe and PTAT current generation. Furthermore by subtracting the positive and negative temperature coefficient signals from one another the effective temperature coefficient is increased and the temperature dependence of the temperature dependent signal is therefore enhanced. Preferably the subtracting comprises applying the positive and negative temperature coefficient signals to a detection node. A positive feedback may also be applied, preferably to the shared bipolar transistor, that is to the transistor used for generating both the positive and negative temperature coefficient signals.
In a related aspect the invention also provides a circuit for generating a temperature dependent signal, the circuit comprising: means for generating a thermal voltage referenced positive temperature coefficient signal using a pair of transistors operating at different current densities; means for generating a transistor voltage referenced negative temperature coefficient signal using a transistor voltage of one of said pair of transistors; and means for subtracting one of said positive and negative temperature coefficient signals from the other of said signals to generate said temperature dependent signal, whereby the temperature dependence of said temperature dependent signal is greater than either of said subtracted signals.